Ancient Lineages of the Keratin-Associated Protein (KRTAP) Genes and Their Co-Option in the Evolution of the Hair Follicle

The study investigates the evolution of keratin-associated proteins (KRTAPs), crucial for hair follicle development. Out of 93 KRTAPs in humans, 53 are linked to a human metallothionein, and 16 to occludin. The metallothionein-linked KRTAPs, which are larger, include high-sulphur and ultrahigh-sulphur KRTAPs, while the occludin-linked set includes tyrosine- and glycine-containing KRTAPs. The study found that metallothionein-linked KRTAPs appeared in increasing numbers as evolution advanced, starting from deuterostomia, with strong sequence similarity found in a sea anemone and a starfish. Occludin-linked KRTAPs arose later, with a homologous KRTAP found in snails. The presence of KRTAP antecedents in non-hairy animals suggests that some KRTAPs may have a physiological role beyond hair fibre characteristics. The researchers propose that these KRTAPs were co-opted by placodes to produce the first hair-producing cells, the trichocytes of the hair follicles.